Recent years have seen a rapid increase in demands for liquid crystal devices in various applications such as word processors, lap-top personal computers, pocket televisions. Particularly, reflective-type liquid crystal device, which displays by reflecting an incident light from the exterior, has been viewed with interest for their beneficial features as to small-power consumption, thinness, and light weight as achieved by eliminating the need of backlight.
Conventional reflective-type liquid crystal devices can be classified into the following three systems: the segment display system, the simple-multiplex driving system, and the active-matrix driving system. In the segment display system, only simple numerals or pictographs for use in, for example, clocks, etc., can be displayed. In the simple-matrix driving system, a complex display can be realized, for use in, for example, personal computers, portable information terminals, etc. In the active-matrix driving system, a display is performed by using active elements such as TFTs (thin film transistors). In any of the foregoing systems, it is preferable to achieve a reduction in power consumption.
In response, Japanese Laid-Open Patent Publication No. 232447/1993 (Tokukaihei 5-232447, published on Sep. 10, 1993) discloses a power consumption reducing technique for the segment display system, wherein in the stand-by state, i.e., in the non-display state of totally white display or totally black display, common electrodes and segment electrodes are set so as to have the same potential, so that the totally white display or the totally black display can be realized under stable conditions. Japanese Laid-Open Patent Publication No. 210492/1990 (Tokukaihei 2-210492, published on Aug. 21, 1990) discloses a technique for reducing power consumption of a driving circuit by setting the impedance of the MOS-type transistor, which directly drives liquid crystals in the stand-by state, in the high impedance state.
The foregoing power consumption reducing techniques are preferable for the segment display type liquid crystal display devices, and figurative performances are therefore limited to the display of simple numerals or pictographs. It is therefore not possible to apply the foregoing techniques to the display devices which deal with complicated information, such as personal computers, portable information terminals, etc.
It is also difficult of apply the foregoing driving method to the matrix-type liquid crystal display devices. Specifically, in the case of 4×4 matrix-type liquid crystal display device as illustrated in FIG. 43, scanning signals to be supplied to scanning signal lines G(0) through G(3) are as illustrated in FIG. 44, and selective voltage is sequentially applied to scanning signal lines G(0) through G(3). At an instant when the scanning line for a certain line is selected in the foregoing manner, data signals are supplied to data signal lines S(0) through S(3) in synchronous with scanning signals, thereby writing charges corresponding to data in respective pixels. Upon completing the scanning of the last line, as illustrated in FIG. 45, after a short period of vertical fly-back time, a scanning operation is started again from the first line.
This vertical fly-back time is initially set to allow electron beams from an electron gun stored in the CRT to return to its original position, and in this regard, such vertical fly-back time is totally unnecessary for the liquid crystal display device. However, in liquid crystal display devices designed for reproducing normal television video images, the vertical fly-back time is set to ensure the compatibility with TV video signals such as NTSC.
As described, for the matrix-type liquid crystal display, it is required to sequentially drive a plurality of pixels in data signal lines arranged in a vertical direction of the screen, and there is no data signal output for driving only one particular pixel corresponding to the segment output of the segment display system. Therefore, even if the data signal line and the counter electrode of a pixel are set in the high impedance state using the drive method of the segment display system after writing charges to pixels in the last line, the charges as written are not held for other pixels than the pixels in the last line. It is therefore not possible to ensure a stable display.
For the matrix-type liquid crystal display of a simple multiplex driving system, although a power consumption is relatively small, for example, in the range of from 10 mW to 15 mW for the size of type 2, the problems arise in basic display performances, such as insufficient brightness, contrast, response speed. In contrast, for the matrix-type liquid crystal display of active driving system adopting TFTs, etc., although sufficient basic display quality can be ensured as to brightness, contrast, response speed, etc., the power consumption within the range of from 100 mW to 150 mW is required for the size type 2it, and it is not possible to achieve a reduction in power consumption to the satisfactory level.
Specifically, for the liquid crystal display device having liquid crystal display elements of active-matrix type adopting the active element, in order to obtain desirable dynamic display, the rewriting frequency which determines the frequency for writing fluctuations in respective pixels, i.e., the rewriting frequency of one screen is generally set to 60 Hz. In the display device of active driving system, rewriting of screen is performed at such high frequency (60 Hz) for the following reason. That is, when performing an impulse type display for displaying one screen utilizing the residual effect of human eyes using a luminescent material which instantaneously flashes light, it is required to rewrite a screen at high speed even when displaying a static image.
In the liquid crystal display device of the active matrix display system, not only that the rewriting is performed at high frequency, but also that the voltage polarity of the data signal is reversed per one scanning signal line, resulting in an increase in power consumption of a scanning signal driver and a data signal driver.
The applicant of the present application performed an experiment of rewriting at a low frequency, i.e., at or below 30 Hz with respect to the liquid crystal device of the Cs-on-gate structure to reduce the power consumption of the liquid crystal display device. As a result, flicker noise occurred. As can be seen from the result of the experiment, it was found that when merely reducing the rewriting frequency in the Cs-on-gate structure to reduce the low power consumption, deterioration of display quality cannot be avoided.
In response, earnest researches have been made to achieve a reduction in power consumption to the sufficient level and a desirable display quality.
For example, Japanese Laid-Open Utility Model Publication No. 50573/1985 (Jitsukaisho 60-50573, published on Apr. 9, 1985) and Japanese Laid-Open Patent Publication No. 10489/1998 (Tokukaihei 10-10489, published on Jan. 16, 1998) disclose the method which realizes reduction in power consumption. These methods are focussed on the method of transmitting a television signal, and reduction in power consumption is realized by stopping the operation of a peripheral driving circuit in the vertical fly-back time, utilizing the feature that the data does not exist in the vertical fly-back period.
Japanese Laid-Open Patent Publication No. 107563/1997 (Tokukaihei 9-107563, published on Apr. 22, 1997) discloses a power consumption reducing technique for the head-mount type display for use in field sequential cubic image display having two liquid crystal panels corresponding to both eyes, wherein the two liquid crystal panels are alternatively driven for each field by not driving one while driving the other.
Further, SID 95 DIGEST pp 249 to 252 and Japanese Laid-Open Patent Publication No. 271795/1991 (Tokukaihei 3-271795, published on Dec. 3, 1991) proposes the multi-field driving method as the method of reducing the power consumption of the TFT liquid crystal driver. In this method, a scanning operation of one screen is divided into several times by scanning the scanning signal lines in one screen by every other line or every other plural lines, and the polarity of the voltage of the data signal line is not reversed while scanning one time. Furthermore, fluctuations in brightness generating each line, flicker noise are cancelled out by the adjoining lines of opposite polarities, and as a whole, a display which is free from flicker noise is realized.
For example, Japanese Laid-Open Patent Publication No. 342148/1994 (Tokukaihei 6-342148), published on Dec. 13, 1994 discloses the method of reducing power consumption by reducing driving frequency (refresh rate) while ensuring memory by adopting ferroelectric liquid crystal to the liquid crystal panel.
However, when stopping the operation of the peripheral driving circuit in the vertical fly-back period, as disclosed also in Japanese Laid-Open Utility Model Publication No. 50573/1985, the vertical fly-back period occupies only 8 percent of the total field period, and the expected reduction in power consumption within this period is only 5 percent.
According to the method of Japanese Laid-Open Patent Publication No. 107563/1997, throughout the field period, any of liquid crystal panels is driven, and the required power consumption may not be increased but cannot be reduced. Furthermore, by adopting the head mount type display for both eyes, either one of the displays is always refreshed, and image without much flicker can be realized. However, for the liquid crystal display device, a display without flicker can be obtained when driving at a frequency of 30 Hz, particularly at above 45 Hz. When applying this method to the system for directly seeing the liquid crystal panel, flicker noise is liable to be perceptible.
Furthermore, flicker noise generates every line even when multi-field driving, and therefore flicker noise is recognized even when the flicker noise is cancelled out between adjoining lines, which leads to a significant deterioration in visibility. Furthermore, an expected reduction in driving frequency is very small, and a sufficient reduction in power consumption cannot be achieved. In the multi-field driving system, one screen is divided into a plurality of sub fields, and scanning is performed very other lines or every other plural lines, and therefore, it is required to read out signals corresponding to the scanning signal line to be driven after once storing the image in the frame memory, and therefore the structure of the circuit inevitably complicated. As a result, the peripheral circuit becomes larger in size, and a manufacturing cost increases.
Furthermore, according to the method disclosed in Japanese Laid-Open Patent Publication No. 342148/1994 (Tokukaihei 6-342148), a ferroelectric liquid crystal is basically for the binary value (black-and-white), and a gradation display cannot be achieved, and a natural display cannot be achieved. Furthermore, in order to arrange ferroelectric liquid crystals in a panel, an advanced technique is required for preparing panels, and thus it is difficult to adopt it in practical applications. Therefore, the foregoing method has not yet being used in practical applications.
In the method of driving a conventional matrix-type display device, it was not possible to realize a reduction in power consumption while ensuring basic display characteristics such as brightness, contrast, response speed, gradation, etc. Furthermore, in the conventional method of driving the matrix-type liquid crystal display device, it is not possible to realize both reduction in power consumption to a sufficient level and a high quality display device without flicker noise at the same time. The foregoing problems are encountered not only in the liquid crystal display devices, but also a generally used matrix-type display devices can be used.
The present invention is achieved in finding out a solution to the foregoing problem, and it is therefore an object of the present invention to provide a matrix-type display device and a driving method thereof which permits the power consumption to be reduced to a sufficient level while ensuring basic display quality such as brightness, contrast, response speed, gradation, etc., can be realized. It is another object of the present invention to provide a matrix-type display device and a driving method thereof which realize reduction in power consumption and high quality display in which flicker noise is fully suppressed.